Interview for the Special Topic of Quantum Computers, July 2010

According to our Special Topics analysis of
quantum computers research over the past decade, the work of
Professor Guifre Vidal ranks at #8 by total cites, with 27 papers
cited 2,751 times.

His record in
Essential Science IndicatorsSMfrom Clarivate Analytics
includes 47 papers cited a total of 3,888 times between January 1, 2000
and February 28, 2010 in the field of Physics. In the
Web of Science®, Vidal's record includes 67
papers cited a total of 4,490 times between January 1, 2000 and June 3,
2010.

Vidal is an Australian Research Council Federation Fellow in the
School of Mathematics and Physics at the University of Queensland in
Brisbane, Australia, as well as a Distinguished Research Chair at the
Perimeter Institute for Theoretical Physics in Waterloo, Canada.

ScienceWatch.com talks with HIM ABOUT HIS
HIGHLY CITED WORK.

Would you tell us a bit about your educational
background and research experiences?

I grew up in Barcelona, where in 1999 I obtained a Ph.D. in Physics from
the University of Barcelona under the supervision of Prof. Rolf Tarrach.

Then I worked for two years in Prof. Ignacio Cirac's group at the
University of Innsbruck, Austria, with a Marie Curie Postdoctoral
Fellowship from the European Community. In 2002 I moved to the United
States to work in Prof. John Preskill's Institute for Quantum Information
at the California Institute of Technology with a Sherman Fairchild
Postdoctoral Fellowship.

Since 2005 I have been a Professor in the School of Mathematics and Physics
at the University of Queensland, in Brisbane, Australia, where I have built
a research group in quantum information, computational physics, and
condensed matter.

What do you consider the main focus of your
research within the area of quantum computing?

A long-term goal of my research has been to better understand quantum
entanglement. Entanglement is a natural consequence of the superposition
principle of quantum mechanics when applied to composite systems. It turns
out that correlations between entangled quantum systems can be stronger
than correlations between classical systems.

I was first attracted to this fascinating subject in the context of quantum
computing, where entanglement is used as a resource for quantum information
processing. However, nowadays entanglement is also intensively studied in
the broader context of quantum many-body physics and has found important
applications both as a natural theoretical framework to study quantum
phases of matter (e.g. in condensed matter physics) and as the key to the
development of new computational tools for strongly correlated systems.

Two of your most-cited papers are the 2000
Physical Review A paper, "Three qubits can be entangled in two
inequivalent ways" (Dur W, Vidal G, Cirac JI, 62[6]: art. no. 062314,
December 2000) and the 2002 Physical Review A paper,
"Computable measure of entanglement" (Vidal G, Werner RF, 65[3]: art.
no. 032314, March 2002). Would you tell us about these papers and why
you think they are so highly cited?

"...while it may still be unclear whether and when it will be possible
to build a quantum computer, it is important to notice that the areas
of quantum information processing and quantum computing have already
produced a lot of useful outcomes."

During my Ph.D. and first years as a postdoctoral researcher, I joined an
ongoing effort devoted to characterizing entanglement in systems made of a
small number of parts, typically just two or three parts. Back then,
entanglement had been identified as a resource for quantum information
processing, including quantum teleportation and quantum cryptography, and
developing a theory of entanglement became a priority.

The aim was to understand how entanglement, as a physical resource, could
be created, transformed, and used in an optimal way. Several notions and
measures of bipartite and tripartite entanglement, as well as pure-state
and mixed-state entanglement, were proposed during this period.

These two papers contributed to that ongoing effort by, respectively,
classifying the possible types of entanglement in a system made of three
quantum bits (or qubits), and by proposing a measure of entanglement for
mixed states that can be easily computed. These results turned out to be
useful milestones in the development of a theory of entanglement.

Another highly cited paper is your 2003
Physical Review Letters paper, "Entanglement in quantum
critical phenomena" (Vidal G, et al., 90[22]: art. no. 227902,
6 June 2003). What does entanglement have to do with critical phenomena
and why is this paper significant?

As I mentioned earlier, the study of entanglement is of relevance well
beyond the context of quantum computing. This paper investigated
entanglement in the ground state of quantum spin chains. The goal was to
establish whether the entanglement between different parts of a many-body
system was enhanced near a quantum phase transition.

We found that, at criticality, entanglement scales in a very simple way
that reveals the universality class to which the quantum phase transition
belongs. These results pioneered the use of entanglement measures to
characterize quantum many-body phenomena and illustrated how tools
developed in the context of quantum computing could be useful in other
areas of research.

It also unveiled connections between the recently developed theory of
entanglement, important computational tools such as Prof. Steven White's
density matrix renormalization group, and previous calculations in
conformal field theory. In other words, it was a good mixture, with the
potential of attracting the interest of researchers with different
backgrounds and skills.

Since then, the study of entanglement in many-body systems, especially in
connection to quantum criticality and topological order, remains very
active.

Oh, no, I did not get tired of quantum computing. At that time it became
clear, however, that new insights acquired in the context of quantum
computing would also lead to significant progress in our ability to
numerically simulate many-body systems. With a few colleagues, including
Prof. Frank Verstraete and Prof. Ignacio Cirac, we started to explore these
possibilities.

"...the study of entanglement is of relevance well beyond the context
of quantum computing."

By using concepts such as quantum circuits and entanglement, we were able
to propose new variational wave functions to efficiently represent the
ground state of many-body systems, or came up with algorithms to simulate
time evolution. These developments were significant because they opened up
a new route to simulating strongly correlated systems.

We must keep in mind that a large number of models proposed to describe
condensed matter systems, including very simple lattice models of
frustrated antiferromagnets and of interacting fermions, remain unsolved
due to the lack of proper computational tools to address them. For
instance, the phase diagram of the Hubbard model for interacting electrons
in two dimensions, which was first proposed in the 1960s and is used to
investigate the metal to insulator transition and cuprate high-temperature
superconductors, still remains highly controversial.

For decades, this lack of numerical tools has slowed progress in several
areas dealing with strongly correlated systems. Our hope is that the new
computational approaches, known as tensor network algorithms, will finally
allow us to answer a number of long-standing open questions, including
perhaps the mechanisms of high-temperature superconductivity.

We have good reasons to remain optimistic, especially after last year's
irruption of fermionic tensor network algorithms for interacting fermions.
It is also encouraging to see that prestigious condensed matter physicists
such as Prof. Xiao-Gang Wen have joined this effort with their own
important contributions.

How has the field of quantum computing changed in
the past decade? Where do you hope to see it go in the next?

Perhaps the original euphoria concerning the feasibility of quantum
computing has been replaced with a more realistic understanding of the long
way ahead of us. Now that the theoretical foundations are largely in place,
we have to wait for further experimental and technological progress.

However, while it may still be unclear whether and when it will be possible
to build a quantum computer, it is important to notice that the areas of
quantum information processing and quantum computing have already produced
a lot of useful outcomes.

Apart from enormously stimulating experimental research, which has led to
impressive progress in our ability to control quantum systems, thinking
about quantum computers has given birth to a new way of looking at quantum
mechanical problems, including a new framework and new tools to address
strongly correlated quantum many-body systems.

Guifre Vidal, Ph.D.
School of Mathematics and Physics
University of Queensland
Brisbane, Queensland, Australia

GUIFRE VIDAL'S MOST CURRENT MOST-CITED PAPER IN
ESSENTIAL SCIENCE INDICATORS: